1. Field of the Invention
The present invention relates to an optical disc recording/reproduction apparatus and method and in particular, to an optical disc recording/reproduction apparatus and method capable of recording or reproducing a data to/from an optical disc with a high density.
2. Description of the Prior Art
In order to record or reproduce a data onto/from an optical disc with a high density, it is necessary to minimize the size of an optical spot applied to the optical disc. The size of an optical spot is known to be proportional to .lambda./NA assuming that .lambda. is a wavelength of a light source and NA is a numerical aperture of an objective lens. Consequently, when the wavelength is constant, increase of the numerical aperture of the objective lens increases the possible recording density. The objective lens used as an aspherical single lens in a currently available optical disc apparatus is considered to have a numerical aperture of 0.6 at maximum because of the manufacturing reasons.
Moreover, as the numerical aperture increases, and as the thickness of the optical disc substrate increases, there will be more affects by the wave font aberration generated by the inclination or warp of the optical disc and assembly accuracy. For this, when using a lens having a large numerical aperture as an objective lens, it is necessary to reduce the thickness of the optical disc substrate. For example, according to the DVD (digital versatile disc) specification, the substrate thickness is defined as about 0.6 mm.
As an objective lens unit exceeding 0.6 of the numerical aperture, there is known a two-lens unit consisting of a solid immersion lens (SIL) and an objective lens which has been suggested by Kino and others (U.S. Pat. No. 5,125,750). With this two-lens unit, there is suggested an optical disc recording/reproduction optical system having the numerical aperture exceeding 0.8.
In this optical system, it is necessary to maintain a distance (air gap) between the SIL and the optical disc at an optimal value. If this air gap is greatly changed, a wave front aberration is generated, lowering the signal quality and, in the worst case, disabling to record or reproduce a data onto/from the optical disc.
In such an optical system using a two-lens unit, to maintain the air gap constant is equivalent to maintain constant a distance between the two lenses of the lens unit. If the distance between the two lenses of the two-lens unit is to be adjusted by an actuator to cope with a change in the thickness of the optical disc, a complicated configuration is required and it becomes difficult to assure an assembly accuracy.